There are two main types of displays used in computer monitors, passive matrix and active matrix. Passive-matrix displays use a simple grid to supply the charge to a particular pixel on the display. Creating the grid starts with two glass layers called substrates. One substrate is given columns and the other is given rows made from a transparent conductive material. This is usually indium tin oxide. The rows or columns are connected to integrated circuits that control when a charge is sent down a particular column or row. The electro-optical material is often sandwiched between the two glass substrates.
A pixel is defined as the smallest resolvable area of an image, either on a screen or stored in memory. Each pixel in a monochrome image has its own brightness, from 0 for black to the maximum value (e.g. 255 for an eight-bit pixel) for white. In a colour image, each pixel has its own brightness and colour, usually represented as a triple of red, green and blue intensities. To turn on a pixel, the integrated circuit sends a charge down the correct column of one substrate and a ground activated on the correct row of the other. The row and column intersect at the designated pixel and that delivers the voltage to untwist the liquid crystals at that pixel.
The passive matrix system has significant drawbacks, notably slow response time and imprecise voltage control. Response time refers to the display's ability to refresh the image displayed. Imprecise voltage control hinders the passive matrix's ability to influence only one pixel at a time.
When voltage is applied to change the optical state of one pixel, the pixels around it also partially change, which makes images appear un-sharp and lacking in contrast.
Active-matrix displays depend on thin film transistors (TFH). Thin film transistors are tiny switching transistors and capacitors. They are arranged in a matrix on a glass substrate. To address a particular pixel, the proper row is switched on, and then a charge is sent down the correct column. Since all of the other rows that the column intersects are turned off, only the capacitor at the designated pixel receives a charge. The capacitor is able to hold the charge until the next refresh cycle; and if the amount of voltage supplied to the crystal is carefully controlled, it can be made to untwist only enough to allow some light through. By doing this in very exact, very small increments, displays can create a grey scale. Most displays today offer 256 levels of brightness per pixel.
Displays that can show colours may have three sub-pixels with red, green and blue colour filters to create each colour pixel. Through the careful control and variation of the voltage applied, the intensity of each sub-pixel can range over 256 shades. Combining the sub-pixels produces a possible palette of 16.8 million colours (256 shades of red×256 shades of green×256 shades of blue). These filters are arranged such that they form vertical red, green and blue stripes across the panel.
The frequency spectrum of radiation incident upon a detector depends on the properties of the light source, the transmission medium and possibly the properties of the reflecting medium. If one considers the eye as a detector the human visual system can sense radiation that has a wavelength between 0.6 nm and 380 nm. Hence this is described as the visual part of the electromagnetic spectrum. Humans perceive certain frequency distributions as having different colours and brightness. A scheme was devised to describe any perceived colour and brightness via adding three basis spectral distributions with various weights. For example in the 1931 CIE colour space any perceivable colour may be described by the following equation:C=xrX+yrY+zrZ Where C is the colour being described, Xr, Yr and Zr are the weights and X, Y and Z are 1931 CIE tristimulis curves which are graphs of the relative sensitivity of the eye Vs wavelength. For any given colour, the weights may be determined by the following equations:
            x      r        =          (              ∫                              C            ⁡                          (              λ              )                                ⁢                      X            ⁡                          (              λ              )                                ⁢                      ⅆ            λ                              )                  y      r        =          (              ∫                              C            ⁡                          (              λ              )                                ⁢                      Y            ⁡                          (              λ              )                                ⁢                      ⅆ            λ                              )                  z      r        =          (              ∫                              C            ⁡                          (              λ              )                                ⁢                      Z            ⁡                          (              λ              )                                ⁢                      ⅆ            λ                              )      
The 1931 co-ordinates are formed via the following normalisation:
            x      r        =                  X        r                              X          r                +                  Y          r                +                  Z          r                                y      r        =                  Y        r                              X          r                +                  Y          r                +                  Z          r                                z      r        =          1      -              x        r            -              y        r            
These may be plotted on the 1931 CIE diagram. The spectral locus defines the pure spectral colours, that is the perception of radiation with a specific wavelength. Colour co-ordinates that are closer or farther from pure spectral colours are described as being more or less saturated respectively. The value of the y coordinate multiplied by 683 is also referred to as the luminance denoted by the symbol L.
The perception model described above accurately predicts that colours on addressable objects can be formed by mixing small areas of three basis colours with modulated intensities which are close in either close spatial or temporal proximity. If the basis colours are plotted on the CIE diagram then the enclosed triangle contains all the colours producible by the system. The enclosed area is called the colour gamut and hence a addressable object with a larger area can addressable object a greater variation in colour and has a greater colour gamut.
Displays employ several variations of liquid crystal technology, including super twisted nematics, dual scan twisted nematics, ferroelectric liquid crystal and surface stabilized ferroelectric liquid crystal. They can be lit using ambient light in which case they are termed as reflective, or backlit and termed transmissive. There are also emissive technologies and reflective technologies such as Organic Light Emitting Diodes and electronic ink which are addressed in the same manner as Liquid Crystal displays.
At present there exist displays that by various means enable the stacking of addressable object planes at set distances. As well as the binocular depth cue, they feature intrinsic motion parallax, where the x and y distance changes between objects displayed on different planes depending on viewing angle. Additionally separate focal planes may be literally be brought in and out of focus depending on the focal length of the lens in the viewers eye. These displays consist of a high-brightened backlight, a rear image panel which is usually an active matrix, colour liquid crystal display, a diffuser, a refractor and a front image plane which are laminated to form a stack. There are generally colour filter stripes as mentioned above, and a black matrix on each display which defines the borders of the pixels. However it should be appreciated that the following discussion applies to all addressable object planes that are addressed by passive or active matrices or have colour filters arranged in any periodic pattern, or any optically active periodic pattern. The displays are close to each other, as far as the viewer is concerned they form two similar, but not identical periodic patterns on the retina. This is because the solid angle subtended by the repeating patterns is different, which causes the colour stripes and black matrix boundaries to have slightly different pitches when projected onto the retina.
These conditions are sufficient to cause a phenomenon called moiré interference, which is characterized by large, annoying vertical red, green and blue stripes. The diffuser combats the interference by spreading the intensity distribution of the image formed by the colour filters. However while this may help remove moiré it has the effect of changing the bidirectional scattering transmission function of the sub-pixels, smearing them to a point spread function thus effectively reducing the resolution of the display. Therefore to make a good display or optical system where the image remains sharp and the amplitude of the moiré interference is hardly noticeable, these two conflicting factors must be carefully controlled.
Typically the diffuser is of the form of a chemically etched series of surface features on a thin (0.000115 meter), birefingent substrate such polyester. If the pattern was viewed under a microscope at 1000× magnification it would be undulating in topology. Because of the polarised nature of the displays this can cause the total luminance, which is evaluated at the front display by the viewer, to be reduced because it changes the degree and polarization orientation from the optimum. A similar pattern is available on a non-birefringent surface such as acrylic but this substrate cannot be made thin enough as not over-blur the rear most pixels. In general one cannot easily control the angular distribution of the light as it exits a typical diffuser. Also because there is an extra layer in the optical stack, extra air-plastic or air-glass interfaces are formed causing back reflections. These decrease the brightness of the display because at least 4% of the light is directed towards the backlight, as opposed, to the viewing direction. The ratio of the reflected and transmitted radiation is given by Fresnel's equations which are well known in the art. Note that if a ray is at some angle from the normal significantly more than 4% of light may be reflected. This reflected light may also be re-reflected out to the viewer, but may not appear to come from the correct origin, reducing the contrast of the display. Also because the film is on a separate sheet it has the tendency to deform due to the heat from the high-brightness backlight which is visible to the viewer and can exasperate the sharpness problem described above. Point spread functions for typical, commercially available diffusers are circularly symmetric, that is their gain is constant for a given radius.
A holographic diffuser is a transparent or translucent structure having an entrance surface, an exit surface, and light shaping structures formed on its entrance surface and/or in its interior. These light shaping structures are random, disordered, and non-planar micro sculpted structures.
These structures are created during recording of the medium by illuminating the medium with a speckle pattern produced in conjunction with coherent light or the combination of incoherent light and a computer-generated mask which simulates speckle. The speckle produced changes in the refractive index of the medium which, when developed, are the micro-sculpted structures. These light shaping structures diffract light passing through the holographic diffuser so that the beam of light emitted from the holographic diffuser's exit surface exhibits a precisely controlled energy distribution along horizontal and vertical axes. Holographic diffusers can be used to shape a light beam so that over 90% (and up to 95%-98%) of the light beam entering the holographic diffuser is directed towards and into contact with a target located downstream of the holographic diffuser. A holographic diffuser can be made to collect incoming light and either (1) distribute it over a circular area from a fraction of a degree to over 100 degrees, or (2) send it into an almost unlimited range of elliptical angles. For example, a 2 degree×50 degree. holographic diffuser will produce a line when illuminated by a LED or laser and a 35 degree×0.90 degree. Thus a holographic diffuser is not a typical example of a diffuser, since it may send most of the incoming light out at elliptical angles and these particular angles may be finely controlled.
The following discussion describes pixel patterns used in the imaging industry. For the purposes of illustration it is assumed a sub-pixel is a 0.1 mm×0.3 mm rectangle, with the long axis of the rectangle in the y direction and a pixel is a 0.3 mm×0.3 mm square, however it should be appreciated that a pixel can be any shape that is possible to tessellate and a sub pixel can be any one of a set of shapes which are possible to tessellate in combination. To define this rigorously consider a set of regular points in 2D space forming a lattice and the same collection of pixels or sub-pixels at these points. Then the pixel pattern is wholly described by the lattice and the collection of sub-pixels or pixels at that point which are called a basis. The lattice can in turn be described by a primitive lattice cell comprised of two linearly independent vectors which form two sides of a parallelogram.
The following radio metric quantities will be used throughout this specification are defined below:
Luminous Flux is the flow rate of visual energy and is measured in lumens.
Illuminance is a measure of photometric flux per unit area, or visible flux density. Illuminance is measured in lux (lumens per square meter).
Luminance is the illuminance per solid angle.                To appreciate the solid angle concept consider a spherical surface of radius r containing an area element ΔA. The solid angle at the centre of the sphere is defined to be        
  ΔΩ  =                    Δ        ⁢                                  ⁢        A                    r        2              .  
Pixels on a transmissive addressable object will be capable of maximum and minimum luminous states. Labelling the maximum state as Lb and the minimum as Ld then the contrast ratio is described by
      C    r    =            L      b              L      d      
The term contrast ratio is usually abbreviated to just contrast.
From http://www.cquest.utoronto.ca/psych/psy280f/ch5/csf.html “The contrast sensitivity function (CSF) plots the contrast sensitivity for the human visual system (1/(contrast threshold)) for all spatial frequencies. Viewers are most sensitive to intermediate frequencies (˜4-8 cycles per degree). Viewers are less sensitive to lower frequencies, and less sensitive to higher frequencies.
The CSF shows us the observer's window of visibility. Points below the CSF are visible to the observer (those are the points that have even higher contrasts than the threshold level). Points above the CSF are invisible to the observer (those are the points that have lower contrasts than the threshold level). The lowest visible frequency (at 100% contrast) is the low frequency cut-off, and the highest visible frequency (at 100% contrast) is the high frequency cut-off.”